Hydrogen is the only energy source with zero carbon dioxide emission. Electrical energy produced from wind, sun and wave power can be converted into hydrogen by the electrolysis of water and the hydrogen produced can be used to generate energy either in proton exchange membrane fuel cells (PEMFC's) or by combustion with the only by-product being water. However, the generation of hydrogen in electrolysers requires an energy input of at least 3.9 kWh/Nm3 [14.04 MJ/Nm3] and then further energy is required to provide hydrogen at a pressure of up to 800 bar [80 MPa]. Moreover, the energy input required can also vary if the power supply fluctuates in level. Prior art alkali water electrolysis cells have a too limited operational range (20 to 100% of the nominal capacity) and cannot provide hydrogen at high pressures, an expensive compression step being always necessary. Electrolysis performed at pressures up to 50 bar (5.0 MPa) provides hydrogen and oxygen typically having a purity of 99.9 mol % and 99.5 mol % respectively. If electrolysis is performed at higher pressures, the solubility of hydrogen and oxygen in the electrolyte, which increases with increasing pressure, results in gases being produced with a reduced purity, the oxygen produced having a lower purity than the hydrogen due to the more strongly increased diffusion of hydrogen to the oxygen side than vice versa. This results in electrolysis having to be performed at a maximum pressure of 32-50 bar (3.2-5.0 MPa), since, although the upper explosion limit (UEL) for hydrogen-oxygen mixtures at room temperature only changes from 95.2 to 95.1 mol % hydrogen from atmospheric pressure to 200 atmospheres, the lower explosion limit (LEL) increases from 4.0 mol % hydrogen at atmospheric pressure to 5.9 mol % hydrogen at 200 bar (20 MPa).
U.S. Pat. No. 2,683,116 discloses the operation of an electrolytic apparatus having a plurality of pressure-resistant cells each containing an individual diaphragm which subdivides its associated cell into an anode and a cathode chamber and having two gas collecting systems, one for the gas spaces of all anode chambers and the other for the gas spaces of all cathode chambers, the method of starting the operation of such apparatus which consists of the following steps, namely (1) filling all gas spaces of both of said collecting systems with nitrogen at an equal pressure of at least two atmospheres abs (0.2 MPa); and (2) thereafter switching on the current to commence electrolysis.
WO 2004/076721A2 discloses a method of electrolyzing water to generate pressurized hydrogen and oxygen therefrom utilizing an electrolyzer comprising one or more electrolyzer cells, the cells individually comprising (i) a cathode of tubular configuration within which a rod-shaped anode is disposed to define an annular-shaped electrolyte chamber between the cathode and the anode, (ii) a separation membrane of tubular configuration disposed within the electrolyte chamber between the cathode and the anode to divide the electrolyte chamber into an anode sub-chamber and a cathode sub-chamber and seal the sub-chambers against gas flow therebetween, the method comprising the steps of (a) introducing an aqueous solution of electrolyte into both sub-chambers of the electrolyte chamber; (b) applying a DC voltage drop across the respective anodes and cathodes of the cells to dissociate water into hydrogen at the cathode and into oxygen at the anode; and (c) separately withdrawing hydrogen and oxygen from the one or more electrolyzer cells preferably further comprising a pressure vessel and generating hydrogen and oxygen at elevated pressure, which elevated pressure is at least 10 psig [0.689 MPa] and particularly preferably including maintaining the pressure differential between the hydrogen and oxygen withdrawn from the cells at not more than about 0.25 psig [17.2 kPa]. This technique is applied in the Avalence Hydrofiller 50-6500-50RG system, but there is still a driving force for the two gases to mix by permeating through the cell membrane. WO 2004/076721A2 discloses that the separation membrane selectively allows passage of liquid but not gas through it and keeps the hydrogen and oxygen gases separated as the generated gas bubbles rise through the liquid electrolyte, but is silent in respect of materials to perform these functions.
US 2010/0187129A discloses a method for producing electrolyzed water, comprising: using an electrolyzing apparatus of water having a structural feature to divide an electrolyzer into an anode chamber and a cathode chamber by a diaphragm, and arranging an anode plate in the anode chamber and a cathode plate in the cathode chamber; carrying out the electrolysis by filling the cathode chamber with water to which electrolyte is previously added; wherein the flow rate of water to be provided to the cathode chamber is restricted to 40 mL/min. [0.67 mL/s] per 1 A of loading electric current or less; wherein the water provided to the cathode chamber is previously softened sufficiently to prevent the formation of scale; and adding non-softened water for dilution with the electrolyzed water produced in the anode and/or cathode chambers to minimize the amount of softened water required to produce electrolyzed water and prepare electrolyzed water sources having desired pH ranges. However, US 2010/0187129A1 is silent in respect of oxygen and hydrogen diffusion or use at high pressures and discloses non-conductive diaphragm materials and is silent in respect of separation membrane materials.
US 2010/0276299A1 discloses a method for increasing the efficiency of a high-pressure [340 to 690 bar (34 to 69 MPa)] electrolysis cell having an anode and a cathode defining an interior portion therebetween, the method comprising: decreasing the current density at the anode and reducing an overvoltage at the anode as the high-pressure electrolysis cell is operated; and decreasing the amount of hydrogen permeation through the cell membrane from the cathode chamber to the anode chamber as the high-pressure alkaline-electrolysis cell is operated. US 2010/0276299A1 particularly discloses high pressure electrolysis cells with separators having cylindrical outer and inner surfaces.
WO 2008/048103A1 discloses an electrolysis device comprising a container having an outer wall, said container being vertically divided into four serial chambers, electrodes being located in the first and last of said chambers, said chambers being separated from each other by semipermeable membranes, wherein the semipermeable membranes are substantially permeable to cations and the semipermeable membranes are preferably substantially impermeable to multivalent cations, a liquid inlet and a liquid outlet being provided to the second chamber, a liquid inlet and a liquid outlet being provided to the third chamber, and a liquid inlet and outlet being provided to the last chamber, said liquid outlet and said liquid inlet being connected to a multivalent cation removal device. However, WO 2008/048103A1 is silent in respect of the diffusion of gas.
This prior art is silent in respect of suitable separators to avoid cross-contamination of the hydrogen and oxygen produced. Separators for use in alkaline water electrolysis cells should be spontaneously self-wettable, ion-permeable, chemically, thermally, dimensionally and mechanically stable and have a low ionic resistance, since the resistance of a separator accounts for up to 80% of the total resistance of an alkaline water hydrolysis cell. Most commercial electrolysers today still use asbestos as a separator. However asbestos is a highly carcinogenic substance. Moreover, the thinnest asbestos diaphragms are 3-4 mm thick, thus limiting the realisable ohmic resistance, and they cannot be used above 85° C. or with aqueous potassium hydroxide concentrations above 30 wt %, making it unsuitable for future applications. A major problem with candidates to replace asbestos as separator materials is their lack of hydrophilicity, their difficult-to-control manufacturing process and their high associated cost. Research work has been carried out on separators based on potassium titanate, polyantimonic acid, polysulphones, hydrophilised polyphenylene sulfide, poly(vinylidene fluoride) (PVDF) and PTFE. None of these materials has been demonstrated to be suitable for future application in electrochemical cells involving the production or consumption of at least one gas.
WO 93/15529A discloses a method for making a porous membrane with a symmetrical structure which is gas-tight when saturated with electrolyte and whereby, according to this method, a solution is made from an organic binding agent in a solvent and the solvent is removed by means of extraction through immersion in an organic non-solvent, characterized in that an amount of metal oxide and/or metal hydroxide is added to the solution. WO 93/15529A further discloses a membrane made according to this method and an electrochemical cell containing said membrane between two electrodes preferably characterized in that it is an alkaline cell and in that the membrane is saturated with electrolyte and thus forms a separator between two electrodes. WO 93/15529A exemplifies separators based upon polysulphone as the binder and zirconium oxide or zirconium oxide and zinc oxide as the metal oxide or hydroxide without using reinforcing polymer supports. Such non-reinforced separators have been commercialized as ZIRFON® separators and exhibit good wettability, low ionic resistance and a high bubble point, but have a typical asymmetric pore structure with finger-like cavities and take 30 minutes [1800 s] to manufacture all of which is unfavourable.
WO 2006/015462A discloses a process for preparing an ion-permeable web-reinforced separator membrane, comprising the steps of: providing a web and a suitable paste, guiding said web in a vertical position, equally coating both sides of said web with said paste to produce a paste-coated web, and applying a symmetrical surface pore formation step and a symmetrical coagulation step to said paste coated web to produce a web-reinforced separator membrane. WO 2006/015462A further discloses a web-reinforced separator membrane, characterised in that the web is positioned in the middle of the membrane and both sides of the membrane have the same pore size characteristics and an apparatus for providing a web-reinforced separator membrane, comprising a web-unwinding station for web-tension control, a spreader roller, a coater with double-side coating with double-sided coating system with automatic paste feeding with vertical (guided) web transportation, and guiding rollers in a heated coagulation bath.
A poster presented by W. Doyen et al. at the World Hydrogen Technologies Convention, held at Montecatini Terme in Italy between 4th and 7 Nov. 2007, reported the development of an advanced separator in three thicknesses (250, 550 and 950 μm) and in two temperature versions (80° C. and 120° C.) for use in high temperature alkaline water electrolysis, referred to as the “NEW-ZIRFON® separator. The “NEW-ZIRFON® separator is reinforced with a polypropylene, ETFE or PEEK fabric and exhibits permanent hydrophilicity, good wettability in strongly alkaline solutions, low ionic resistance (0.13 Ω·cm2 in 6M KOH at 70° C. for the 550 μm thick version), capability of operating at current densities up to 10 kA/m2, no dimensional changes, a tensile strength of at least 25 MPa, a symmetric pore structure, a total porosity between 50 and 55%, a bubble point above 7 bar (0.7 MPa) and a double skinlayer with identical pores at both sides (mean value 0.08 μm) thereby offering a double safety for preventing the mixing of gases. Double skinlayer means a separator with two denser layers (with pores with a diameter smaller than 0.1 μm) at its two outside surfaces, one each side of the separator (one at the upper side, the other at the bottom side). Between these both layers there is a solid layer (more than 80% of the thickness) with much more open pores with a diameter of between several microns to a maximum of 10 μm. However this intermediate layer is not an open space channel with low hydraulic resistance for electrolyte circulation/passage. Its resistance for flow passage is so high that it is not useful for the “free” circulation of electrolyte. W. Doyen et al. also discloses that the continuous vertical double-sided coating process disclosed in WO 2006/015462A1 is capable of manufacturing 50 cm wide separators.
WO 2009/147086A1 discloses an apparatus for producing an ion-permeable web-reinforced separator comprising a duplex type impregnating apparatus comprising two slots each with upper and lower slot faces, said faces having a vertical orientation or an orientation which may deviate from vertical by no more than 10°, for providing premetered quantities of a dope simultaneously to either side of an elongated porous web, said quantities on both surfaces are identical or may deviate from identical by no more than 5%, a transport means providing for downwards transport of said elongated porous web through said duplex impregnating apparatus, said downwards transport having a vertical orientation or an orientation which may deviate from vertical by no more than 10°, and subsequent phase inversion, coagulation and washing stations, said phase inversion station providing for phase inversion of said dope and said coagulation station providing for coagulation and washing of solvent from the resulting phase-inverted dope, wherein there is an air gap between said duplex impregnating apparatus and said phase inversion station and wherein the distance between the lower faces of each impregnating apparatus is greater than the distances between the upper faces of each impregnating apparatus. WO 2009/147084A1 discloses a process comprising the steps of: (i) providing an elongated porous web, said elongated porous web comprising two outermost surfaces; (ii) transporting said elongated porous web downwards between two impregnating heads [6] and [6′] comprising two slots each with upper and lower slot faces, said faces having a vertical orientation or an orientation which may deviate from vertical by no more than 10°, parallel to said elongated porous web providing simultaneously to both surfaces of said elongated porous web metered quantities of a dope, said quantities on both surfaces are identical or may deviate from identical by no more than 5%, comprising at least one membrane polymer and at least one solvent therefor; (iii) thereby impregnating said elongated porous web completely with said dope and providing dope layers on each surface of said outermost surfaces of said elongated porous web with an equally thickness or a thickness which may deviate from equally by no more than 5%, said thickness being independent of the gap between one of said lower slot faces and the surface of said elongated porous web nearest thereto; (iv) subjecting said dope associated with said elongated porous web immediately after dope-impregnation to phase inversion with at least one non-solvent wherein said phase inversion of said dope layer is symmetrical on each surface of said web, thereby forming a membrane; and (v) removing residues of said at least one solvent for said at least one membrane polymer from said membrane thereby producing an ion-permeable web-reinforced separator, characterised in that said dope is shear-thinning. Example 1 exemplifies the double-sided coating of the 3D spacer fabric FC 360/50PW with metal oxide-containing dope and discloses that the substantially hollow by-pass channel between the continuous regions of the fabric is also filled with dope. Applications of the ion-permeable web-reinforced separators were envisaged in batteries e.g. in non-aqueous secondary batteries that employ a lithium-containing transition metal oxide as the positive electrode, a lithium dopable/dedopable carbon-based material as the negative electrode and a non-aqueous electrolyte solution as the electrolyte solution (lithium ion secondary batteries); in fuel cells; and in electrolytic or electrochemical cells e.g. in local hydrogen generators in which the hydrogen is produced by the electrolysis of water.
Separators on the basis of the technology of WO 2009/147084A1 and WO 2009/147086A1 in which the integrated permeate channel is filled with dope have been commercialised by AGFA-GEVAERT N.V. as ZIRFON® PERL separators for alkaline water hydrolysis as replacement materials for chrysotile asbestos and PPS cloth. Moreover, it is claimed in its publicity material dated July 2009 that such separators allow for highly efficient cell operation at high current densities with high durability.
EP 1625885A1 discloses a membrane, comprising a permeate channel consisting of a 3D spacer fabric having an upper and a lower fabric surface spaced apart by monofilament threads at a predefined distance, said permeate channel being interposed between two membrane layers, wherein said membrane layers are linked at a multitude of points with said upper and lower fabric surfaces. EP 1625882A1 discloses that the membrane layer preferably comprises a hydrophilic filler material selected from the group consisting of HPC, CMC, PVP, PVPP, PVA, PVAc, PEO, TiO2, HfO2, Al2O3, ZrO2, Zr3(PO4)4, Y2O3, SiO2, perovskite oxide materials, SiC; and an organic binder material selected from the group consisting of PVC, C-PVC, PSf, PESU, PPS, PU, PVDF, PI, PAN, and their grafted variants, but no materials are exemplified. As applications for such membranes MBR, microfiltration, ultrafiltration, membrane distillation, pervaporation, vapour permeation, gas separation, supported liquid membranes and pertraction were included. Although EP 1625885A1 is silent in respect of porosity in general and pore size and bubble point in particular, microfiltration and ultrafiltration both require materials with 50 to 80% porosity, but it contains no hint or indication that such membranes could function as a separator. W. Doyen et al. disclosed at Achema an innovative back-washable flat sheet membrane envelope having as key elements the use of 3D spacer-fabrics as supporting and permeate drainage structure and membrane layers directly coated on both sides thereof with the hollow by-pass channel between the faces being used for permeate collection or as a drainage chamber, see FIG. 1. During coating filling up of the hollow by-pass channel is avoided by using a specially developed textile, an adequate dope viscosity and an appropriate coating process.
Kerres et al. in 1996 in Desalination, volume 104, pages 47-57, describes evaluation of microporous polymeric membranes just produced from poly(ethersulphone)s UDEL®, RADEL R®, RADEL A® and VITREX® in advanced alkaline electrolysis and reported that these membranes meet all requirements as diaphragms, like low resistance connected with sufficiently high pressure stability to avoid gas intermixture in the electrolysis compartments, although these cells had no long-term stability. Furthermore, Lu et al. in 2007 in Journal of Membrane Science, volume 300, pages 205-210, report the application of a homogeneous blend membrane made from poly(ether sulphone) and poly(vinylpyrrolidone) to alkaline water hydrolysis.
Although the use of ZIRFON® PERL separators in alkaline water hydrolysis result in highly efficient cell operation at high current densities with proven long-term stability, permanent hydrophilicity, small pore size, symmetric pore structure and reinforcement by an open mesh fabric (ETFE, PP etc.), such separators do not offer a solution to the problem of the cross contamination of the hydrogen and oxygen produced by alkaline water electrolysis at high pressures.